The present invention relates to a semiconductor laser diode, and more particularly, relates to a semiconductor laser diode suited for uses where high power output and long life-time are required, such as excitation light source for optical fiber amplifiers.
Development of optical information-processing techniques and optical communication techniques in recent years is remarkable. It is to a fact, for instance, the realization of many advanced technical achievements such as high-density storage system by a magnet-optical disc and bidirectional communication by optical fiber networks. Especially in the field of communication, many studies are being made on large-capacity optical fiber transmission lines which can genuinely cope with the coming age of multimedia communication and rare earth (such as Er.sup.3+, etc.)-doped optical fiber amplifiers (EDFA) for amplification of signals having flexibility for such transmission system. As a matter of course, development of a high power output and long life-time semiconductor laser diode, which is an essential component of EDFA, is looked forward.
There exist, in principle, three different emission wavelengths of a semiconductor laser diode applicable to EDFA, that is, 800 nm, 980 nm and 1,480 nm.
It is known that, when viewed from the standpoint of the amplifier itself, excitation at 980 nm is desirable in consideration of gain, noise figure and other factors. Since the important key point of the laser having this wavelength of 980 nm is coupling efficiency to the optical fibers, it is desired that the laser output especially its transverse mode should be stable even if laser diode has changing of current injection, temperature, reflection from the fiber. Further, being used as an excitation light source, this laser diode is required to realize high output and long life-time.
There is a request for application of this laser as SHG light source in this range of wavelength, and thus development of a high-performance laser for use in the above-mentioned and various other fields of application has been desired.
However, the hitherto reported semiconductor laser diodes having a wavelength around 980 nm, especially diodes having a structure designed for coupling to an optical system have involved the following problem. In the high-output region, for example, in the light output region of around 150 to 200 mW, there is observed a phenomenon of non-linearity, called "kink", in the current and light output characteristics. This is attributable to unstability of the transverse mode due to various causes and detrimental to stable coupling to fibers. This kink is also observed from a comparatively low light output region in the lasers having the similar layer structures, when the layer composition or thickness slightly differs. The above phenomenon is even seen from a particularly low light output region when the temperature is high.
As a result of extensive studies for solving the above problem, it has been found that in a semiconductor laser diode with an InGaAs system having an index guided structure, the refractive index step in the horizontal direction and the width of the current injection region for narrowing current injection pass necessary for laser oscillation have a strong correlation with the kink level and the temperature characteristics, and by optimizing the defined ranges of these parameters and their combination, laser diode with both a high kink level and good temperature characteristics has been realized. On the basis of the above, the present invention has been achieved.